Cordless telephone systems generally consist of a stationary base unit, connected to the telephone network, and a plurality of mobile remote units linked to the base unit by a communications highway, such as an FM radio link. Once communications have been established between the base and the remote, the user is able to operate the remote in the same manner as a conventional telephone.
The advantage of a cordless telephone system over a conventional telephone is of course the mobility of the remote unit. The radio link between base and remote allows the telephone subscriber to move anywhere within radio transmission range, rather than being restricted to the length of the standard telephone cord. Such mobility has substantial advantages over a conventional telephone, particularly in large home or office environments.
Due to the advantages inherent with a cordless telephone system there have been numerous prior art attempts to make such a system suitable for widespread application. Prior art cordless telephone systems are shown, for example, in U.S. Pat. Nos. 3,335,406; 3,644,681; 3,806,663 and 4,053,717. Notwithstanding these prior art attempts to produce a useful and practical cordless telephone, several problem areas remain.
Two important considerations in any cordless telephone system are establishing and maintaining the communications link between remote and base and conserving battery power at the remote unit. In a typical cordless telephone system the communications link is initiated by the remote user activating the remote unit, (by lifting the handset for example) which in turn, enables the transmission of a carrier signal to the stationary base unit. Reception of the carrier signal at the base unit activates a relay which serves to connect the base unit to the telephone network. In order to maintain the connection to the telephone network it is necessary to continuously transmit the carrier signal from the remote unit. Continuous transmission not only wastes power at the remote but, more importantly, severly limits the number and range of the remote units. That is, with a carrier transmission technique, each remote must transmit on a separate carrier frequency, resulting in expensive multiplication of receiver circuitry at the base. Alternatively, if a single carrier is used, the transmitter must have a very limited range so as to limit interference with adjacent remote units. Limiting the range of transmission destroys the cordless telephone's primary advantage, mobility, while multiple carrier signals make the base and remote expensive and impractical.
It is therefore an object of the present invention to provide a cordless telephone system which does not require the transmission of multiple carrier signals to maintain a communications link beteen base and remote units.
It is a further object of the present invention to provide a cordless telephone system which does not require limited range transmitters to prevent interference between cordless telephone units.
Conversation of battery power at the remote unit is also of primary importance. The use of battery power is, of course, necessary for remote unit mobility, but repeated battery changes are expensive and undesirable. Battery power is typically conserved by supplying power only to the radio receiver at the remote unit, when the remote unit is not being used, and supplying power to the remainder of the remote unit circuitry when the remote unit is activated. Although this basic approach is desirable for battery conservation, the manner in which a remote unit is generally activated, through use of an externally mounted switch, can lead to accidental remote unit activation and loss of battery power.
It is therefore another object of the present invention to prevent accidental activation of the remote unit and thereby prevent accidental loss of battery power.